Superheterodyne receiver



SUPERHETERODYNE RECEIVER Filed Feb. l5, 1931 3 Sheets-Sheet 1 -i-ISUV -lv l INVENTOR n//LL//w ,4. MACa/v/:w

n @Y "n, il! lhluwuuldp/wl mio ATTORNEYS /Pa /1 r/ Vf ANH /F/of 770A/ Oct. 4, 1932.

w. A. MacDoNALD 1,881,235

SUPERHETERODYNE RECEIVER Filed Feb. l5, 1931 3 Sheets-Sheet 2 .@fg b S k w C a 1 l Ej 1 l soo ,ooo |500 D: 50o :ooo |500 FREUENCK-/f/LCFCLES PEI? SECO/V5 INVENTQR WMZ/AMA. MACO/VL ATTORNEYS Oct. 4, 1932. Wl A MaCDONALD SUPERHETERODYNE RECEIVER Filed Feb. 13, 1951 JMA vvv

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INVENTOR W/z/AM AMAQMA/ALU MMM ATTORNEYS Patented Oct. 4, 1932 .UNITED STATES PATENT OFFICE WILLIAM A. MACDONALD, OF LITTLE NECK, NEW YORK, ASSIGNOR TO HAZELTINE CORPORATION, A CORPORATION OF DELAWARE SUPERHETERODYNE RECEIVER Application led February 13, 1931.

This invention relates to radio signaling and more particularly to radio receivers of the superheterodyne type.

The. principal objects of this invention are to obtain in a superheterodyne typeof radio receiver, high sensitivity, selectivity, uniformity of amplification, freedom from crosstalk and interfering noises and whistles and negligible radiation of oscillations, over a wide range of frequency.

To these ends, provision is made for the use of a radio-frequency oscillator, the output voltage of which may be made to be substantially uniform, or to vary in a desired manner over the required tuning range of frequencies, the chosen type of variation to be determined by the characteristics of the receiver. Further provision is made for uniformly amplifying and selectively transmitting through the receiver all the important frequencies of the side bands of the received signal.

The usual superhcterodyne receiver arrangement comprises an antenna and ground, or a loop antenna, a radio-frequency-ampl1fier provided with signal selecting circuits, a modulator or first detector, a local oscillator for producing local oscillations which are combined with the incoming carrier signal frequency at the modulator to produce a new carrier signal wave of intermediate frcquency, an intermediate frequency amplifier, a second detector for producing the modulation component of the intermediate frequency waves, an audio-frequency amplifier, and a signal translating or sound producing system.

It has long been known that the conventional type of antenna and radio-frequency amplifier system, employing as coupling circuits ordinary tuned radio-frequency transformers having comparatively few turns on the primary and a larger number of turns on the secondary Winding thereof, is characterized by a response, or amplification, which is considerably greater at the higher than at the lower portion of the tuning range of frequencies.

The conventional type of local oscillator ordinarily employed in a superheterodyne receiver to beat with the incoming signal Serial No. 515,528.

carrier-frequency delivers an Output voltage which also is considerably greater at the higher i frequencies than at the lower frequencies to which the oscillator is tuned.

The beat or intermediate frequency voltage output of the modulator, the frequency of which is ordinarily the difference between the radio-carrier frequency and the local oscillator frequency, is proportional to the product of the voltages of the radio-frequency signal and of the local oscillator impressed upon the modulator within the normal operating limits of the devices. Hence, in a superheterodyne receiver of the conventional type, the effect of the radio-frequency amplifier and of the local oscillator is cumulative, that is, the magnitude of the intermediate frequency output voltage of the modulator rapidly increases with increasing frequency, when considered relative to a fixed voltage of the signal received by the antenna or loop.

It is desirable, from an operating standpoint, to maintain the response, or amplification of the portion of the receiver from the antenna up to and including the first modulator, substantially uniform over the operating range of frequencies. In accordance with the present invention there is provided a local oscillator having its circuit constants so proportioned with respect to the characteristics of the antenna and radio-frequency amplifier circuits that the output voltage characteristic of the oscillator comv pensates for non-uniformity of amplification of the amplifier; that is to say, the output voltage of the oscillator decreases with increased frequencies to compensate for the rising output voltage of the conventional ty e amplifier at the higher frequencies, whereiiy the overall response of the receiver is main.- tained substantially constant.

The oscillator may, of course, be given any desired predetermined output characteristic; the desired characteristic will be determined by the characteristics of the particular apparatus employed. For example, if the radio-frequency amplifier includes a coupling system of the type describedin U. S. Patent o. 1,763,880, granted to lCarl E. Trube, June 10th, 1930, the response of the amplifier may be substantially uniform over the tuning range of frequency. In such case it will usually be desirable to provide an oscillator having a uniform voltage output over the frequency range of the oscillator.

It is to be preferred that the radio-frequency system be highly selective, by virtue of which extraneous signals are `highly attenuated, so that the tendency toward undesired beats in the modulator is further reduced.

An important feature of the invention is the revision of an intermediate fre uency amp ifier which selectively and uni ormly transmits the intermediate carrier frequency and the associated sidebands. The intermediate frequency amplifier comprises one or more vacuum tube amplifier stages coupled by an arrangement of coupling' circuits which act to provide the above noted desirable transmission characteristic. One of the interstage coupling circuits comprises a plurality of syntonously tuned circuits coupled by a degree of coupling greater than optimum, whereby the transmission of that particular coupling system is characterized by resonances suficiently spread to provide a transmission band which readily transmits all the frequencies of the sidebands. Since this type bf coupling system provides a somewhat decreased transmission over a range of frequencies between the resonances, other of the interstage coupling systems are proportioned to have a single resonance peak which lies between the resonance peaks of the firstmentioned coupling system, whereby the cumulative effect upon the entire intermediate-frequency amplifier is to provide a substantially uniform transmission over the entire sideband range of frequency.

The interstage coupling systems of the intermediate-frequency amplifier which are characterized by a single resonance peak, preferably each comprise a transformer hav ing a primary winding and a secondary winding coupled sufficiently close so that a condenser connected across one of the windings tunes the system as a whole to the intermediate frequency. Itis preferable, although not essential, that the winding which is so tuned is the anode circuit, or primary winding, since the input impedance of the coupling system is thereby made relatively high andl great attenuation of undesired signals is obtained. The other winding of the transformer is preferably naturally resonant below the range of radio-frequency signals to be received, that is, it is capacitively reactive to frequencies in the broadcast range.

A feature of the intermediate-frequency transformer construction is the adjustment of the voltage step-up ratio'to a desired value. This involves providing 4the proper number of secondary turns to ive the desired ratio, while at the same time maintaining the natural resonance of the winding at a desired value below the tuning range of frequency, by furnishing the necessary effective capacity. To secure this re uired effective capacity a proper form actor should be chosen for the winding which will provide a distributed windingcapacity of the desired value; if necessar the winding capacity may be supplemented y an external or added capaeity.

T'he combination of a sharply selective radio-frequency amplifier and the intermediate-frequency amplifier having the broad. u niform, transmission characteristic is particularly advantageous since it permits a moderate amount of Zmistracking of the received si al frequency and of the local oscillator requency without seriously inipairing the quality of transmission through.

the intermediate-frequency amplifier.

An effective means for furnishing the desired oscillator characteristic comprises a vacuum tube oscillator having an anode circuit which is coupled to the 'grid circuit by a dual .element coupling system, one of the coupling elements of which tends to produce an output voltage which increases at higher frequencies, the other of which tends to cause the output voltage to decrease at the higher frequencies. By the proper relative proportioning of these two coupling elem'ents any desired output characteristic may be obtained within wide limits.

In addition to the advantage of enabling a desirable response-frequency characteristic to be obtained, this type oscillator affords the additional advantage of generating a smaller amplitude of harmonics, particularly, at the higher frequencies, than the conventional type oscillator. A reduction of harmonics is an important item in a superlieterodyne receiver since harmonics, if present to an appreciable degree, beat with extraneous signals in the modulator, and any of these beats occurring within the intermediate frequency transmission band duces disturbing notes.

It is contemplated that the oscillator shall have a relatively small degree of coupling between the grid and anode circuits; by virtue of the small coupling impedance, small changes in the constants of the tube produce only slight changes in the generated frequeiicy.

Other features relate to the shielding of the radio-frequency apparatus and connecting leads for the purpose of preventing radiation therefrom.

The above and other features will more fully appear from the following detailed description when read in conjunction with the drawings, of which:

Fig. 1 illustrates a superheterodyne radio receiver the component electrical units of prowhich are arranged in accordance with this '.uvention;

Fig. 2 illustrates response characteristics of the radio-frequency amplifier and of the oscillator of the receiver ofFlg. 1;

Fig. 3 illustrates transmission characteristics of the intermediate-frequency amplifier and of portions thereof:

Figs. 4 and 4A show a. construction of an intermediate-frequency transformer adapted to provide a relatively large voltage amplification;

Figs. 5 and 5A show a construction of an intermediate-frequency transformer adapted to provide a smaller voltage amplification than the transformer of Figs. 4 and 4A;

Fig. 6 illustrates a superheterodyne receiver embodying the invention but which differs in some respects from the arrangement of Fig. 1, and which is shown complete with a source of power supply;

Fig. 7 shows graphically the types of response characteristics obtainable in the receiver of Fig. G; and

Fig. 8 shows in cross section a coupling device employed in such a receiver.

Fig. 1 illustrates a complete superheterodyne receiver embodying features of the present invention. The receiver includes an antenna circuit l() coupled to a radio-frequency amplifier designated in general as 11. The radio-frequency amplifier 11 comprises a vacuum tube amplifier 12 of the four-electrode, or screen-grid, type which comprises a cathode 13, an anode 14, a control electrode, or grid 15, and a screen-grid 16 partially surrounding the anode. The amplifier tube 12 is coupled to the antenna through a radio-frequency transformer 17, the primary winding 18 of which is connected in the antenna circuit and the secondary winding 19 of which is connected to the grid 15 of the amplifier 12. The coupling transformer 17 is tuned to the signal frequency by means of aA variable condenser 2O shunted across the secondary winding 19.

The output of amplifying tube 12 is coupled to a vacuum tube modulator tube 21, also of the four-electrode typo, comprising a cathode 22, an anode 23, a control electrode, or grid, 24, and a screen-grid 25. The grid, or input circuit, of the modulator is coupled to the anode of the radio-frequency amplifier 12 by a radio-frequency transformer 26, the primary winding 27 of which is connected to the anode 14 and the secondary winding 28 of which is connected to the grid 24. The coupling transformer 26 may be similar to the antenna circuit transformer 17, and is similarly tuned by a variable condenser 29 shunted across the secondary Winding 28.

The receiver is provided with a local oscillator system 30 comprising a three-electrode vacuum tube 31 and associated circuit elements which are so proportioned as to cause the tube 31 to produce oscillations of a desired frequency. The oscillator system constitutes a contributing feature of the present invention and will be subsequently described in greater detail. At this point it is sufficient to state that the oscillator system is provided with a variable tuning condenser 94, and an output coil 32 which is connected in the input circuit of the modulator 21 by virtue of its connection between the modulator cathode 22 and ground.

The anode circuit of the modulator 21 is coupled to an amplifier designated generally as 40, which is adapted to transmit a frequency band which is lower in the frequency scale than the signal frequencies transmitted by the radio-frequency amplifier. Since the frequency band transmitted by the amplifier 4() lies between the audio-frequency range and the radio-frequency tuning range, this amplifier is called an intermediate-frequency amplifier. The intermediate-frequency amplifier comprises two screen-grid vacuum tubes 41 and 42 which are the same type as the radio-frequency amplifying tube 12. The first intermediate-frequency amplifier tube 41 is coupled to the modulator 21 through a coupling system comprising two similar tuned circuits 43 and 44. One of these tuned circuits 43 which is connected in the anode circuit of the modulator, includes an inductance 45 and a capacity 4G: and the other of the tuned circuits 44, which is connected to the grid of amplifier 41, includes an inductance 47 shunted by a capacity 48. The inductances 45 and 47 are coupled magnetically.

The second intermediate-frequency amplifier tube 42 is coupled to the first intermediate-frequency tube 41 by an intermediatefrequency transformer 49, the primary coil 50 of which is tuned by a fixed condenser 51, and the secondary winding 52 of which is shunted by a resistance 53. An adjustable tap 54 connects a point of the resistance to the grid of tube 42.

The output of tube 42 is coupled to the input of a detector tube 6() by means of a coupling transformer 55 which may be similar to transformer 49. As the intermediatefrequency amplifier constitutes another feature of the invention it will be dealt with in greater detail subsequently.

The second detector is of the two-electrode type commonly known as a Fleming valve; although it is shown in form as a three-element tube having a cathode 61, a plate 62 and a grid 63, the cathode and plate are connected together to constitute a single cathode, so that the tube is in effect a twoelectrode detector having a cathode and an anode, the grid acting as an anode in this case. The input of the detector is coupled to the output of the intermediate-frequency ICS 'well-known push-pull relation.

amplifier by a connection between the high potential .end of the secondar of transformer and the anode in t is case the grid) of the detector and y a connection from the low potential end of the said secondary winding, through a resistance 65, to the cathode. The resistance is shunted by a. condenser 64 for providin a low impedance path for the interme iate frequency signals around resistance 65. n

An audio-frequency amplifier, designated generally as is connected to the detector circuit in the following manner: A res1stance 66 is connected at one end to the point between resistance 65 and the secondary winding of transformer 55. The other end of resistance 66 is connected through a blocking condenser 67 to one end of a potentiometer, or tapped resistance 68, the other end of which is connected to the cathode 61 of the detector. The input potential for the audio amplifier is the voltage between the low potential end of resistance 68 and the tap.

The audio-frequency amplifier 70 comprises three stages of amplification. The first two stages include respectively amplifying tubes 71 and 72, resistance-coupled 1n tandem in a conventional manner by shunt resistanees 76 and 77 and blocking condenser 78. The output of tube 72 is coupled to -the last audio amplifying stage, which comprises a pair of tubes 73 and 74 connected 1n the The output of the push-pull stage is coupled to a loud speaker 75.

The receiver is adapted to be tuned by a uni-control arrangement; this is effected by operating variable condensers 20, 29, and 94 from a single shaft; this operation is represented by the dotted lines 56 and 57.

The sources of operating potentials such as the filament heating sources and the grid, screen-grid and anode potentials, are not shown in the drawings. These potentials may be supplied by any of the well-known methods. There are indicated in the drawings potentials which are well adapted for application to the various leads; these lpotentials are given with respect to ground potential.

For the purpose of automatically controlling the strength of the signal current delivered to the audio amplifier, there is provided a volume controlling syst-em which automatically regulates the amplification of the receiver so that the detected, or audio, signals remain substantially uniform. The volume controlling arrangement is of the type described in a paper presented before the Institute of Radio Engineers by H. A. Wheeler and published in pages 30-34 of the Proceedings of the Institute of Radio Engineers, January, 1928. The system comprises a connection extending from the lower end of resistance 66 to the control electrode 15 of radio-frequency amplifier 12 and to the control electrode of intermediate-frequency amplifier 41. The connection 80 is led to the control electrodes of amplifiers 12 and 41 by connections to the low-potential ends of the grid circuit windings 19 and 47, respectively, of the associated coupling systems. There are included in the connection 8O resistances 81 and 82, the function of which will be more fully explained later. For the purpose of keeping the grid potential from the cathodes, but still enabling the grid circuits to be completed, there are provided blocking condensers 83 and 84. The above-described automatic volume control system is disclosed and claimed in the copending applications of H. A. Wheeler, Serial No. 203,879, filed July 7, 1927, and Serial No. 502,972, filed December 17, 1930.

The receiver is provided with a number of resistors some of which furnish biasing potentials for vacuum tube grids, and others of which are inserted in the leads supplying operating potentials to the electrodes of the tubes. There are also provided by-passing condensers at advantageous points. These elements contribute toward good operation; and since they are in general use and are well-known in the art, no further details are given here.

The following is a brief description of the operation of the receiver: A radio signal received by the antenna 10 is selected in the well-known manner by the selective circuits of coupling transformers 17 and 26, which are tuned to the same frequency. The modulated carrier signal, after being amplified in the radio-frequency amplifier is impressed upon the grid circuit of the modulator 21. The oscillator 30 is tuned in conjunction with the selective circuits 17 and 26 to apply to the modulator a frequency which differs from the signal frequency by a desired amount. The oscillator frequency may be either greater than, or less than, the radio carrier frequency, but it is preferably greater than the radio frequency. By virtue of the Wellknown phenomenon of modulation, there is produced in the output of the modulator a new carrier frequency which is equal to the difference between the frequency of the received radio signal and the local oscillator frequency. This difference frequency is commonly known as the intermediate carrier frequency, since it is lower than the radiofrequency of the received signal but is above the audible range. The intermediate carrier frequency has associated with it the side band frequencies with which the signal is modulated.

The selective coupling circuits of the intermediate-frequency amplifier are adjusted lto freely transmit the intermediate carrierfrequency and the associated side bands and to effectively exclude all other signals. The

i manner and are converted amplified output of the intermediate frequency ampli er is impressed upon the twoelement detector 60, inthe output ofnwhich there appears the modulation component, that is, the audio-frequency signals. The audio-frequency signals are .amplified in the audio-frequency amplifier in the well-known into sound by the loud speaker connected to the output of the audio amplifier.

The following is a brief outline of the operation of the volume controllingI circuit:

When a modulated carrier signal is impressed upon the two-electrode detector, there appears across resistances 65 and 66 a voltage havin gtWo components, one an audio-frequency component and the other a direct current component. The direct current component is proportional to the strength of the received carrier signal. An increase of the received carrier signal has the effect of increasing the voltage across resistances 65 and 66, that is, of causing the-.potential of point to become more negative with respect to ground.

ASince the potential at point 85 is impressed through the connection 80 upon the grids of amplifying tubes 12 and 41, the effect is to renderthese grids more negative when the` signal strength increases. Likewise when the signal strength decreases, the grids of amplifiers l2 and 41 become less negative. Due to the variation of the potential of the am plifier grids in this manner, attendant upon the variation of the received signal strength, the amplification increases when the signals are weak and decreases when the signals are strong, so that the net effect is to maintain the signals at the second detector substantially uniform in strength. The audio-frequency component of the detected signal is prevented from appearing at the grids of amplifiers 12 and 41 by Virtue of the filtering action of resistances 81 and 82 and condensers 84 and 83.

The magnitude of the intermediate frequency voltage at the output of the modulator 21 is expressed by the relation:

Ei kE13E0 Where E, is the intermediate frequency voltage at the output of the modulator;

Es is the radio-frequency signal voltage at the input of the modulator;

E0 is the voltage of the local oscillator at the input of the modulator; and

k is a proportionality constant.

It is generally desirable to maintain constant the amplification of the portion of the receiver between the antenna and the intermediate frequency amplifier, that is to maintain constant the product ESRC; It is contemplated to provide this feature of uniform amplification by means of a novel combination of the local oscillator system 30 and the radio-frequency amplifier. The elements of the oscillator system are so proportioned with res ect to the response characteristic of the ra io-frequency amplifier, that the oscillator voltage E., varies in a manner complementaryi ID to the variation of the amplified radio-frequency signal voltage E whereb the product EvEo and hence the voltage i, is maintained substantially constant; or if desired,

Ei may be 4given any desired characteristic.-l `Y6 Referring now specifically to the oscillator system, 1t comprises a three-electrode oscillat ing vacuum tube 31 havin a cathode 90, an

anode 91 and a control gri 92. The grid circuit includes an oscillatory circuit compris-,Slt

ing an inductance 93 and a variable capacity 94. The inductance 93 is connected at one end to the rid 92 and at the other end through the paral el arranged capacity 95 and reaf parallel arranged resistance 97 and capacity 98. The function of the resistance 97 is to provide a biasing potential for the grid.

For the purpose of establishing the condition of oscillation, the anode 91 is connected through a coil 99 to the point between the grid circuit inductance 93 and capacity 95. The coil 99 is so situated relative to coil 93 that a substantial degree of inductive coupling exists between the two coils. By virtue of this circuit arrangement of the oscillator there exist in common with both the grid and the anode circuits of the oscillator, the capacity 95 and the mutual inductance M of coils 93 and 99. These common, or mutual impedances arey made sufliciently large so that the tube 31 is set into oscillation; and the frequency of the oscillation is the frequency at which the circuit including inductance 93 and capacities 94 and 95 is resonant.

Since the voltage across the capacity 95 is least at the highest frequencies of the range over which the oscillator is tuned, and is greatest at the lowest oscillator frequency, it follows thatv the effect of this capacity in pro'- ducing an oscillating voltage is greater at the lower frequencies than at the higher frequencies. It further follows that the eect of the mutual impedance M in producing an oscillatory voltage is greater at high frequencies than at lower frequencies.

The oscillators commonly employed heretofore in superheterodyne receivers have been of the conventional type, that is, a simple feed-back coil has been used to transfer energy from the anode circuit to the grid circuit. As already observed, however, this form of oscillator is characterized by an output voltage which increases in magnitude when the oscillator is tuned to higher frequencies; which is usually an undesirable characteristic. By means of the present oscillator, however, it is possible to obtain any desired output characteristic within wide limits. Assume, for example, that there is employed the present type of oscillator system, and that the output voltage at the higher frequencies is greater than desired. Accordingly, if the mutual inductance be decreased relative to the mutual capacity, as by removing turns from coil 99, the output voltage of the oscillator is reduced at the higher freuencies to a greater degree than at the lower requencies. By properly proportioning the mutual inductance and mutual capacity, any desired output voltage-frequency characteristic may be obtained; and hence any desired response-frequency characteristic may be imparted to the receiver.

The radio-frequency coupling systems 17 and 26 may be transformers having primary windings of relatively few turns and secondary windings of many more turns. The effect of employing such transformers in an amplifier is to cause the amplifier to amplify highfrequency carrier signals to a much greater extent than lower frequency carrier signals. The amplification-frequency characteristic of such an amplifier is illustrated graphically by curve a of Fig. 2. Fig. 2 is a graph, the ordinates of which represent the ratio of maximum to minimum amplification and the absciss of which represent the frequency of radio-frequency carrier signals. The type of transformer indicated by curve a provides an amplification which is almost three times greater at a frequency of 1500 kiloeycles per second than at 500 kilocycles per second. The oscillator output, then, in order to compensate for this characteristic of the radio-frequency amplifier must be given a characteristic of the type indicated by curve b of Fig. 2. This type of oscillator output characteristic may be obtained by properly proportioning the relation between the mutual inductance and the mutual capacity of the oscillator circuit, in the manner previously described. The overall characteristic due to the combination of the amplifier having the characteristic (a) and the oscillator having the characteristic (b) is represented by curve (c) which is'substantially constant over the tuning range. It should be understood that these curves represent the law of variation of the individual responses rather than the relation of one response to another. For it will be clear that while the magnitude of the oscillator response characteristic may be fixed, the

i magnitude of the response to the incoming signal depends upon the strength of the particular signal being received. This assumes, of course, that the automatic volume control effect upon the radio-frequency amplifier alone is not made absolute; some of the automatic volume regulation takes place in the first intermediate-frequency amplifier in the receiver of Fig. 1. Likewise, the magnitude of the response characteristic (c) is dependent upon the proportionality factor c.

Although a conventional type transformer ma be employed to couple the antenna to the ra io-frequency amplifier, it is preferable to employ a coupling system of a type in which the transformer comprises a helical secondary winding wound on a cylindrical core, and a compactly wound primary winding of a large number of turns, located near one end of the secondary winding. The primary coil is preferably wound in the opposite direction to the windings of the secondary coil, the directions of the windings being considered for this purpose relative to the low-potenti al terminals. The effect of this type of transformer is to render the antenna circuit inductive over the entire broadcast range of frequency, whereby the Voltage amplification is maintained substantially uniform over that range. The above described coupling system is described and claimed in my copending application Serial No. 280,464, filed May 25, 1928, and the transformer per claimed in my copending application Serial N o. 498,785, filed November 28, 1930.

If the antenna coupling system is not of the uniform response type, the oscillator will have to be proportioned to compensate for this irregularity in addition to compensating for the non-uniformity of the radio-frequency amplifier.

In addition to enabling the amplification characteristic of the portion of the receiver ahead of the intermediate frequency amplifier to be adjusted to provide uniform amplification over a broad range of tuning frequencies, the use of an oscillator circuit of the type employed in the receiver of Fig. 1, provides additional advantages over oscillator circuits previously employed.

One of these advantages lies in the fact that when the oscillator circuit is proportioned to provide an output voltage which is constant or which increases at lower frequencies, the effective coupling between the grid and anode of the oscillator is relatively small at the higher frequencies to which the oscillator may be tuned. The reason for the small dcgree of coupling at the higher frequencies is that the reactance of the coupling condenser becomes small at these frequencies. This 1s especially important because'the smaller the coupling betwen the grid and plate circuits,

se is disclosed and the smaller is the effect of the tube constants upon the generated frequency; or expressed in another manner, changes in the tube constants such as the mutual conductance or plate resistance, produce a small or negligible effect upon the generated frequency.

It is helpful to be able to adjust the coupling between the grid and anode circuits of the oscillator to the value that will produce optimum coupling and the desired output voltage at the highest frequency to which the creased at lower oscillator will be tuned; for it is in the highfrequency range that very small changes in the electrical constants of the oscillator produce relatively large changes in the generated frequency.

At the lower frequencies, a given change in the constants produces a smaller effect u on the oscillator frequency than at the hlgher frequencies. Consequently, at the low-frequency range of operation the eiective rid-anode circuit coupling maybe made muc range and even though there may be appreciable changes in the tube constants at the low-frequenc range, the effective change in the generate frequency is smaller.

A second major advantage, and an important reason for the use of the oscillator circuit of the type descrfbed, is the small amplitude of the harmonic frequencies produced. It has been found that the harmonics produced by an oscillator of the conventional type may be much greater at the high-frequency range of operation than at the low-frequency range. In the conventional type of oscillator system, the large magnitude of the harmonic frequencies at ,the high-frequency range is accentuated by the fact that in order to cause the tube to oscillate at the' low-frequency range, it is necessary lto provide a coupling between the grid and anode circuits which is substantially -over-optimum at the high-frcquency range. This relatively large coupliug at the high-frequency range produces a higher output level than at the low-frequency range; and since the harmonics produced occur at high exciting voltages, it has been found that the magnitude of the harmonic frequencies generated by the conventional type of oscillator may be much higher at the high than at the low-frequency range By employing an oscillator-circuit of the type of the present invention, it is possible to employ substantially optimum coupling between the grid and anode circuits of the oscillator over the entire frequency range of operation, or, if desired, optimum coupling may be provided at the high-frequency range and the coupling im edance automatically inequencies. Although the magnitude of the harmonic frequencies increases when the grid-anode coupling is increased, and also as the output level is increased, there is an opposing action which tends to reduce the magnitude of the harmonies at the lower frequencies. This opposing action results from the increased size of the tuning capacity at the lower frequencies.

By a pro er choice of the signal level at the input o the modulator, that is, by providing suilicient radio-frequency amplification, so that a very large oscillator voltage is not required to provide a given intermediate frequency voltage at the output of greater than at the high-frequency.

` latoi` circuits, with a. fixed the modulator, and by causin the output voltage of the oscillator to remain uniform or to decrease with increased frequenc 1t 1s possible to cause the magnitude of t e harmonic frequencies to remain substantially uniform over the entire range of operation and at a sufliciently low level so that they do not seriously interfere with the normal performance of the receiver.

The electrical constants of the elements associated with the oscillator 'system 30 have been so chosen as to provide unieontrol operation between the radio frequency and oscilrequency difference between these circuits. This result is accomplished by the proper choice of the inductance 93 and the capacity 95. The capacity 95 in conjunction with the inductance 93 and the mutual inductance M between coils 99 and 93 regulate the slope of the output characteristic of the oscillator. The capacity 95 therefore functions in the dual sense to proportion the output level and also to align the oscillator circuits with the radiofrequency tuner circuits to provide a constant frequency difference between these systems.

The intermediate-frequenc amplifier is so designed that there is provi ed a fairly uniform transmission of all the frequencies of the side bands of the intermediate-carrier frequency, and furthermore, means are prog vided for insuring that no frequencies otheri than those of the desired signal are transmitted to any substantial degree.

The coupling system which couples the output of the modulator to the input of the intermediate-frequency amplifier is of the double-tuned type, that is, it comprises a pair of syntonously tuned circuits coupled electromagnetically, one of the tuned circuits 43 being situated in the modulator anode circuit and the other tuned circuit 44 in the grid circuit of the Iirst intermediate-frequency amplifier. The degree 'of magnetic coupling is preferably somewhat over-optimum so that the transmission band of this coupling system is somewhat broadened by virtue of the pair of slightly spaced resonant peaks which it is well known are obtained by this arrangement. The frequency spread of the resonance peaks should preferably Vbe about equal to or greater than the frequency range of the side bands with which the intermediate carrier-frequency is modulated The two succeeding intermediate-frequency transformers 49 and 55 are preferably identical, although this identity is not essential. These transformers-are so constructed that there exists a close electromagnetic coupling between their primary and secondary windings, whereby the coupling system tunes as a Whole at the resonant frequency of the winding which is shunted by the. condenser. Each of the two trasformers 49 and 55 is characterized by a single resonance, rather than a double resonance such as is obtained from the first intermediate-frequency coupling system.

There is an advantage from a commercial standpoint, as well as from a transmission standpoint, in providing one coupling system of. the double-resonance type and the other coupling system of the single-resonance t pe. It has been found diicult to adjust the oul ble-resonance system commercially so that the two resonance peaks are properly spaced and are of approximately the same height. Hence, it is desirable that there be only one of these double-resonance systems. The advantage from the transmission standpoint is that the single-resonance of the transformers 49 and 55 can be made to lie midway between the peaks of the double-resonance coupling system, whereby theover-all transmission provided by the intermediate frequency amplifer is substantially .uniform over the side band range of frequencies. Y

Curve a of Fig. 3 illustrates the transmission-frequency` characteristic 'of the double-resonant system. The ordinates represent the ratio of the gain at resonance to the gain near resonance and the absciss represent frequencies in kilocycles per second. The frequency marked zero represents the intermediate carrier-frequency, and the absciss on either side of the carrier-frequency are the frequencies of the side bands.

Curve b of Fig. 3 represents the transmission-frequency characteristic .ofthe transformers 49 and 55. The combination of the two types of coupling system cooperates to produce a transmission characteristic of the type illustrated in curve ci of Fig. 3. i The intermediate frequency amplifier can be readily constructed so that the over-all response side-band range.

at a side-band frequency of four kilocycles from the intermediate carrier-frequency isas much as 80% of the response at the carrierfrequency.

ByV providinga relatively fiat-topped respouse characteristic of the type o curve c, slight mistracking between the oscillator frequency and radio-frequency carrier frequency, that is, variations in the difference between the radio-frequency carrier and oscillator frequency, cause no'serious change in the over-all sensitivity over the important In considering an intermediate-frequency amplifier of the type under discussion includ.- ing coupling systems utilizing double tuning between the output of one tube andthe input of the second tube, together with coupling systems utilizing but a single resonant circuit tuned to the intermediate frequency, it has' been found preferable, although not essential, to arrange the coupling systems utiliz;

lng but a single physically resonant circuit so that the resonant circuit is connected beto tune the secondary stages in this way, for suchconnections tend yto attenuate voltages of signal frequency more rapidly than when the tuned circuit of the resonant unit transformer is connected between the cathode and control grid.

Conditions of design, however, may re uire the physically tuned resonant circuit o the intermediate frequency coupling system to be connected between the cathode and control grid. If this be the case, then, the full advantage of band selection can be .secured in the intermediate-frequency amplifier, bu't there may be a greater tendencyffor interfering signals on nearby channels to be transmitted directly through the intermediatefrequency amplifier, thus producing interfering beats `by the second detector.

It is usually desirable to tune the primary circuit` of transformer rather than the secondary circuit because this transformer operates into a two-electrode type of detector, namely, detector 60.. This form of detector imposes a. shunt load on the transformer, so that if the Secondary circuit were tuned instead of the primary circuit, the effect would be to materially impair the resonance characteristic of this transformer.k By tunin the primary circuit and employing a stepown transformation ratio, the impedance of the secondary circuit may bevarranged to match the impedance of the system into which it feeds, thus producing the most efficient condition of operation. i If for somel design reason it is necessar rather than the primary circuit, thenpthe detector may be connected across a portion of the tuned secondary, the impedance of which is approxi- 'matelyrequal to they impedance of the load circuit. y

It is important in the design of the intermediate-frequency ycoupling v transformers that the natural period of the rwinding' which is nlqt tuned by a physical capacity shall have suc urally resonant at a `frequency lower than the lowest ybroadcast vfrequency to be received. The factors which determine the resonant frequency of this -windin are: its natural inductance, the distribute capacity of the winding and the capacity of the devices connected across itsfterminals. All of these elements must be considered in the selection of the proper resonant frequency such that the winding is always capacitively reacted to frequencies in the broadcast band.

If this precaution is not followed and the untuned winding is arbitrarily chosen so that its natural period falls within the broadcast electrical constants that it will be nat- Y uof the receiver. .If it band then it has been found thatrthere is usually a high order of amplification to broadcast frequencies, and voltages o'f signal frequency which would normally be of small ma. itude roduce undesirable heterodyne whistles in t e receiver.

The following values have been found satisfactory in the design of an intermediatefreqliency coupling transformer, although all of t e values may be modified within wide limits and still be within the scope of the present invention:

Intermediate frequency=175 kilocycles per second Inductance of primary winding= 8.99 millihenries Inductance of secondary win'ding=5.16 millihenries Coupling coefficient between primary and secondary windings=37% Capaclty across primary winding=100 micro-microfarads approx.

The above inductanee values are obtainable by winding on a one-half inch core in bobbins one-quarter inch wide, a primary coil of 800 turns No. 38 double silk-covered copper wire and a secondary winding of 600 turns of No. 38 double silk-covered wire. The bobbins containing the primary and secondary coils are co-axially placed side by side and are enclosed by a shielding can of 1%-inch diameter.

When the transformer is constructed in accordance with the above specification the resonant period of the secondary winding is about 350 kilocyeles per second. This value of the resonant frequency is not necessarily the best value for all receivers; the best value for the resonant frequency will depend somewhat upon the particular design be desired to make the resonant period higher than the value of about 350 kilocycles per second, the number of turns on the secondary winding should be somewhat less than the value given in the above table; or, on the other hand, if it be desired to reduce the number of secondary turns and still maintain the resonant frequency at about 350 kilocycles per second, the coil should be constructed to have a higher distributed capacity, in accordance with methods outlined in succeeding paragraphs.

It has been found convenient to regulate the amplification of the intermediate-frequency amplifier by means of the turns ratio assigned to the transformer, the resonant frequency of the secondary winding being adjusted to the desired value by providing the proper value of distributed capacity. If a large gain be desired, the number of secondary turns should be made relatively large and the secondary winding so shaped that it has a relatively low distributed capacity. Factors which tend to reduce the distributed capacity are small wire size, thick insulaer is connected has a tion over the wire and a thin pancake form of coil.

An intermediate-frequency having a low distributed ca acity is illustrated in Figs. 4 and 4A; iv. 4 being an elevation view in section, and Fig. 4A being an end view. In these figures, the core is a cylindrical form 100, the primary winding is a pancake form of coil 101, and the secondary winding is a thin pancake coil 102. The coils are preferably wound with wire havin heavy insulation.

f, however, it is desired that the transformer have a. lower amplification than the type just described, the secondary winding transformer may be composed of fewer turns of a heavier wire having thin insulation, and the form of the windings may be such that the axial length is relatively large and the Iradial thickness small. Such a transformer of low amplification and high distributed secondary capacity is illustratedrin Figs. 5 and 5A, in which 103 represents the core, 104 the primary winding, and 105 the secondary winding.

Itis usually preferable to construct the intermediate frequency transformer so that the value of the effective capacity required to shunt one of the windings, and thereby tune the transformer to the intermediate frequency, is fairly large, that is, somewhere in the order of 100 micro-microfarads; by virtue of this large capacity, slight changes in the input or output capacities of the associated vacuum tubes will not materially affect the resonant frequency of the transformer.

It is inconvenient from a mechanical and,

from an economic standpoint to construct an adjustable capacity of the order of 100 micromicrofarads. A form of external condenser which has been found satisfactory comprises a pair of metal leaves about one inch square separated by a dielectricy of mica or other suitable material and provided with a supporting frame and a compression screw which may alter the spacing between the metal leaves, thereby providing an adjustment of the capacity value. such a condenser isA of the order of 40 micromicrofarads. If a larger value of capacity is desired, it is necessary to either increase the size of the leaves or to add additional leaves. Either of these expedients for providing the increased capacity requires an extremely rugged construction in order to prevent warping of the leaves; which would result in changing the natural period of the transformer and thereby materially affect the sensitivity.

To permit the use of the smaller condenser of about 40 micro-microfarads, it is preferable to construct the transformer so that the winding across which this external condenshigh distributed capac- The capacity value of' ity, in accordance with the previous discuss1on.

It has been found that greater sensitivity is required in certain localities than in other locallties. To enable the receiver to meet any of the required conditions of sensitivity, one of the intermediate-frequency transformers is provided with an impedance shunted across its secondary winding so that the sensitivity may be adjusted to any definite value. This is the resistance 53 of Fig. 1 having the adjustable tap 54. It is preferable to provide two or more definite ta s having a switch for selecting any one o them, rather than to employ a potentiometer of the slide-wire type.

Fig. 6 illustrates a superheterodyne receiver in accordance with the present invention which varies in`details from the receiver of Fi 1. The potentials required to energize t e vacuum tubes of the receiver are obtained from a source of rectified, filtered alternating current. The radio-frequency amplifier is provided with three selective circuits instead of two selective circuits as in the receiver of Fig. 1. The antenna is coupled to the first radio-frequency amplier tube 116 by a preselector composed of two tuned circuits. The first of these' tuned circuits comprises a transformer 110, the secondary winding of which is tunable by a variable condenser 111. Transformer is preferably constructed in a form similar to that of transformer 17 of Fig. 1, previously described. The primary winding of transformer 110 has shunted across it a high resistance 112 provided with a variable tap connected to ground, the pur se of this variable resistance being to provi e a manual volume control.

The second selective circuit of the preselector comprises an inductance 113 and a variable condenser 114. The second selective circuit is coupled in tandem with the first selective circuit by means of a small inductance coil 115 attached at one end to the loW- otential end of the secondary coil of trans ormer 110 and at the other end to the low potential terminal of inductance 113. The coupling is effected by virtue of mutual inductance M which exists between coils 115 and 113. This second selective circuit is connected to the input of amplifier tube 116.

The third selective circuit of the radio-frequenc amplifier is the coupling system 117 electrically located between the. output of amplifier 116 and the input of the modulator 118. This coupling system is of the type disclosed in United States Patent No. 1,763,380

issued June 10, 1930, to C. E. Trube. This b coupling system comprises an input circuit including the anti-resonant combination of fixed condenser 120 and fixed inductance 121, in series with an inductance 122. Inductance 121 and condenser 120 should be antiresonant aty a frequency which is slightly below the lowest frequency of the broadcast range. The output circuit of the ycoupling system 117 comprises an inductance 123 which is tunable to the signal frequency by a varivable condenser 124. The inductance 123 is magnetically coupled to inductance 121 and to inductance 122. The effect of such a coupling system is to cause the amplifier toA rovide an amplification which is substantially uniform over the tuning range of frequency; this effect is described in greater detail in the above-noted Truhe patent.

The radio frequency amplifier of Fig. 6 possesses a two-fold advantage over the radio-frequency amplifier of Fig. 1. One of these advantages is the greater selectivity afforded by virtue of the three tuned circuits instead of only two tuned circuits. This results in greater attenuation of signals of undesired frequencies, whereby whistles and other undesirable noises due to the presence of undesired frequencies in the receiver are substantially eliminated.

Because of the great attenuation of undesired frequencies, the magnitude of image frequency signals received at the modulator is very small. By the term image frequency is meant the frequency which added to, or subtracted from, the oscillator. frequency would produce in the modulator an intermediate-frequency signal of the same frequency that the local oscillator frequency combined with the received signal frequency produces in the modulator.

The two selective circuits of the radio-frequency amplifier in Fig. 1 attentuate the image frequency lto a low value, but, of course, the attenuation is not as great as in the radiofrequency amplifier of Fig. 6. Consequently, the amplifier of Fig. 6 is preferable in localities in which many interfering signals are present; but in other localities, where there is comparatively little interference, the selectivity of the radio frequency amplifier of Fig. 1 is adequate.

The other advantage of the radio-frequenc amplifier of Fi 6 is that amplification of t e amplifier can ie made substantially uniform over the broadcast range of frequency.

The local oscillator system 125 is substantially the same as the local oscillator of the receiver of Fig. 1; hence, no further discussion of it is required here except to state that since the radio-frequency amplifier is characterized by a response which 1s uniform over the tuning range of frequency, the condition required to provide over-al1 uniform amplification is that the oscillator circuit shall e so proportioned that` its output voltage is uniform throughout its range of operation. If desired, of course, the over-all amplification may be given any characteristic desired by pro ortioning the oscillator circuit according y in the manner previously described.

1,ss1,ass

Fig. 7 illustrates graphically the typles of response characteristics obtainable in t e receiver of Fig 6. The ordinates represent relative amplification and the absciss represent frequencies of the tuning range. Curve (a) indicates the over-all response at the output of the modulator, which is obtained when boththe radio-frequency amplifier and theoscillator response characteristics are also of the type of curve (a), that is, uniform with frequency. Curve (b) shows the over-all response at the output of the modulator when the radio-frequency amplifier response is uniform but the oscillator response is made to decrease at the higher frequencies. In this case the oscillator response is also of the same general type as that indicated by curve (b).

The receiver of Fig. 6 is provided with a single intermediate-frequency amplifier tube 126, the input of which is coupled to the modulator 118 by a coupling system 150. The output of amplifier 126 is coupled to a detector tube 127 by a coupling system 151.

The circuit arrangement of the coupling system 150 is similar to that of the coupling system between modulator 21 and intermediate frequency amplifier 41 of Fig. 1. The coupling system comprises a pair of coupled inductances 152 and 153, respectively tuned to the intermediate frequency by condensers 154 and 155. The system 150 may be constructed in the same manner as is the similar system of Fig. 1; it is often preferable, however, for the purpose of easily obtaining a correct adjustment, to proportion the elements of coupling system 150 in the manner to be presently described.

Fig. 8 illustrates in cross-section the construction and assembly of such a coupling system. The arrangement comprises the coils 152 and 153 random wound, or layer wound, respectively in bobbins 157 and 158. The bobbins are mounted coaxially on a form, or core, 159, which is adapted to be fastened to a base, or a chassis, by brackets 160 and 161. Surrounding the bobbins containing the coils is a cylindrically shaped shielding ring 162, adapted to be fastened to the base at the 4lower end. The shielding ring, which is preferably of heavy copper or aluminum or other metal having low specific electrical resistance, is located in proximity to the coils so that there exists a substantial coupling between the ring and the coils.

The effect of the shield is to cause the effective coupling between the coils to be reduced to a lower value than would exist if the shield were absent. It is possible, therefore, to place the coils fairly close together so that there exists a higher degree of coupling than is required for the proper performance of the coupling system; the shielding ring is then so placed relative to the coils that the effective coupling between the coils is reduced to the required value. The coupling system'of Fig. 8 and the transformer structure thereof are described and claimed in United States Patents 1,855,054 and 1,855,055, issued April 19, 1932, to J. Kelly Johnson.

The coupling system 151 may be similar to coupling system 49 of Fig. 1, except that the fixed tuning condenser 163 is shown shunted across the secondary winding instead of the primary winding, as in the case of Fi 1.

The detector tube 127 is of the three-e ectrode type instead of the two-electrode type shown in Fig. 1. There is shown no automatic volume controlling circuit such as is shown in Fig. 1. The three-electrode detector could be used with 'a volume controlling circuit if desired, but it is usually preferable to employ a two-electrode detector for this purpose.

The audio-frequency amplifier is a pushpull stage comprising tubes 128 and 129. The output of the push-pull stage is coupled to an electro-dynamic type of loudspeaker 130.

The energizing potentials for the entire receiver are supplied by a power supply system 131 which is adapted to be operated from an ordinary commercial source of low-frequency alternating current by means of a conventional socket plug 132. The drawings illustrate a conventional arrangement of a power transformer 133, a full wave rectifier 134, a filter 135 and all the resistors for providing the necessary operating potentials for the various tubes of the receiver. The field winding 136 of the loudspeaker is also operated from the power set.

In any radio receiver employing an oscillator there is a tendency 'for radiation to occur which may seriously interfere with neighboring receivers. 'l he radiation is caused by'the electrostatic and electromagnetic fields of the oscillator tuning coil and of exposed portions of the oscillator leads connecting the various elements of the 'oscillator system. Additional radiation occurs from other elements of the receiver because of their association with the oscillator system. There is some radiation from the elements and leads of the modulator since the oscillator voltage is impressed upon the modulator input circuit. Some radiation is likely to occur from the common connections in grid and anode power supply sources of the oscillator tube, and of other tubes of the receiver, so that the oscillator frequency is likely to be induced into the power line, from which it may be radiated.

It has been found that by means of adequate shielding it is possible to reduce all radiation to such a small degree that other receivers located only a few feet distant are substantially unaffected by interfering siging the oscillator coil by means of a grounded metallic enclosure of ylow specific resistance, such as copper or aluminum. This is shown in Fig. 6 as the grounded shield 137. The oscillator tuning condenser and the oscillator tube are also shielded by means of grounded metal enclosures 138 and 139, respectively.

. It is preferable to mount all the apparatus lso on a shallow chassis pan and to have all of the physically large elements such as tubes, coils and tuning condensers mounted on top of the pan. All the connecting leads from the various component elements should be brought through holes in the pan and all connections made beneath the pan. Since certain of these leads carry radio-frequency currents of relatively large magnitude which would tend to radiate, a metal bottom is provided for effectively shielding all leads.

Second order radiation effects resulting from the introduction of the oscillator voltage into the input of the modulator are prevented by providing appropriate grounded shields 140 and 141 for the modulator tube and for the inductances of the coupling system at the input of the modulator.

Radiation is further suppressed by isolating the oscillator circuit from the common power supplies as far as radio-frequency currents are concerned. Thev principal isolation required is the use of a separate automatic biasing resistor 142 in the cathode circuit of the oscillator and proper filtering for the anode potential supply.

The oscillator voltage delivered to the modulator tube should be prevented from affecting other elements of the receiver by similar isolation means; these include separate filtering of the grid-biasing circuit, the screengrid circuit and the anode circuit of the modulator.

It is advisable to shield the radio-frequency tube and the associated circuit elements and connections, to prevent radiation from these sources; these shielding means are shown in Fig. 6 as shield 143 around tube 116, and shields 144 and 145 around the inductances of the preselector.

If the previously described precautions as to shielding have been observed, it is usually unnecessary to provide filtering in the power circuit leads. Where it is found necessary, however, to filter the power circuit prior to rectification, this may be accomplished by inserting inductance coils 146 and 147 in series with the power line and a pair of shunt capacities 148 and 149 in series across the line, the midpoint between the capacities being grounded. Although the shields are illustrated only in Fig. 6, it should be understood that they are equally applicable and useful 'in the receiver of Fig. 1.

What is claimed is: 1. In a radio receiver, in combmation, a radio-frequency amplier characterlzed by a response which rises with increased frequency, an oscillator havin a response which decreases with increased rcquency, a modulator for modulating the radio frequency signall and the oscillator output voltage,

the response characteristic of said oscillator being complementary to the response characteristlc of said radio-frequency amplifier whereby the modulated response of said modulator is substantially unlform over the frequency range of operation.

2. In a radio receiver, a radio-frequency amplifier, a modulator and an oscillator, said oscillator comprising a vacuum tube having a grid circuit and a plate circuit, said grid and plate circuits being mutually coupled by two coupling impedances having different variations of impedance with frequency, whereby the output of said oscillator may be automatically adjusted in accordance with the amplification characteristic of said radio-frequency amplifier to produce a response from said modulator which is greater at lower frequencies than at higher frequencies.

3. A combination in accordance with claim 2 in which the grid and plate circuits of said oscillator are coupled both capacitively and magnetically.

4. In a radio receiver, a radio-frequency amplifier, a modulator and an oscillator, said amplifier having an output signal voltage which varies with frequency and said oscillator having an output voltage variation which is complementary to that of said amplifier, whereby the product of the signal voltage and the oscillator voltage at said modulator is substantially constant over the frequency range of operation.

5. A signal response system including a radio frequency system adjustable over a frequency range, an oscillator-modulator system, an amplifier system tuned to the difference frequency of the oscillator and radio frequency systems, elements connected in the output circuit of said modulator of such characteristics as to respond most efiiciently to said difference frequency, said adjustable radio frequency system being characterized as having a voltage response which is substantially uniform over its adjustable range, said oscillator being characterized as having an output voltage characteristic which rises with decreased frequency, whereby the resultant voltage developed across the output circuit of said modulator rises with de creased frequency.

6. A signal response system including a. radio frequency system tunable over a frequency range, an oscillator-modulator system, an amplifier system tuned to the difference frequency of the oscillator and radio frequency systems, and elements connected in the output circuit of said modulator of such characteristics as to respond most efficiently to said difference frequency, said tunable radio frequency system being characterized as having a response which increases with increased frequency over its tunable range, said oscillator being characterized as having an output voltage which rises with decreased frequency whereby the resultant voltage developed across the output circuit of said modulator remains substantially uniform over said tunable frequency range.

7. A signal response system including a tunable signal-frequency amplifier of the vacuum tube type, having a cathode, anode and control electrode, a modulator-oscillator system comprising avacuum tube having a `grid and plate, and a second amplifier system, said second amplifier system being most responsive to frequencies corresponding to the difference frequencies between the oscillator and signal frequencies, means connected in the output of said modulator for transmitting said difference frequencies to said second amplifier, coupling means associated with said signal-frequency amplifier, elements of said coupling means being connected between said anode and cathode and being characterized by the fact that they are capacitively reactive to at least a portion of the frequencies in the tunable range. and inductively reactive to the said difference frequencies, so that the output of said signal-frequency amplifier is uniform over said tunable range, said oscillator including a pair of coupling impedances between the grid and plate circuits thereof, characterized bv an output characteristicl which is substantially uniform over its output range, whereby the resulting voltage appearing across the output of said modulator is substantially uniform over the adjustable range of frequency of said first mentioned amplifier.

8. A signal response system including a tunable radio-frequency amplifier of the vacuum tube type having a cathode, anode and control electrode, a modulator-oscillator system and a second amplifier system, said second amplifier system being most responsive to frequencies corresponding tothe difference between the oscillator and signal frequencies, said oscillator having grid and plate circuits. means connected in the output of said modulator for transmitting sai d dif'- ference frequencies to said second amplifier. coupling means associated with said radiofrequency amplifier, elements of said coupling means being connected between said anode and said cathode and being characterized by the fact that they are inductively reactive to all frequencies in the tunable range and also inductively reactive to all frequencies corresponding to said difference frequency, so that the radio-frequency voltage at the output of said coupling means increases at higher tuning frequencies, said oscillator circuit including a pair of coupling impedances between the grid and plate circuits adjusted to cause the output of said oscillator to fall when tuned to higher frequencies, whereby the resultant voltage of said difference frequency at the output of said modulator remains substantially uniform over said adjustable frequency range.

9. In a radio receiver, in combination, means for receiving radio signals, a local oscillator system and a modulator for modulating the received signals with the output of said oscillator system, said modulator comprising a vacuum tube having a control circuit, said oscillator system comprising a vacuum tube having a cathode, a grid and an anode, a connection including a first inductance in series with a capacity between said grid and said cathode, a connection including a second inductance in series with said capacity between said anode and said cathode and said first and second inductances being mutually coupled, and a third inductance coupled to at least one of said first two inductances, said third inductance being included in the control circuit of said modulator.

10. In a radio receiver, in combination, a radio-frequency amplifier having a selective circuit which includes a transformer having a tuned secondary winding and a primary winding of fewer turns than said secondary winding, whereby said amplifier is charactcrized by a response which is greater at the higher frequencies to which said circuit is tuned than at the lower frequencies, a modulator, and an oscillator system comprising a vacuum tube having a cathode, an anode and a grid, the anode and grid circuits of said oscillator being mutually coupled by a capacitive impedance and an inductive impedance, said mutual coupling impedances eing so related to each other that said osl cillator impresses upon said modulator a voltage which varies over the frequency range of operation in a complementary manner to the response of said amplifier.

11. In a radio receiver, in combination, a, radio-frequency amplifier having a, selective coupling system which causes the amplification of saidamplifier to be substantially uniform over the tuning range of frequency, an oscillator system comprising a vacuum tube having a cathode, an anode and a control electrode, the anode and grid circuits of said oscillator being mutually coupled both inductively and capacitively, said' capacitive and inductive couplings being so related to each other that the output voltage of said ocsillator is substantially uniform over the operating rangeof frequency, and a modulator in which said signal voltage and said oscillator voltage are combined.

In testimony whereof I afiix my signature.

WILLIAM A. MACDONALD. 

